Cleaning aluminum surfaces



United States 3,041,259 Patented June 26, 1962 3,041,259 CLEANING ALUMINUM SURFACES William B. Stoddard, Jr., Matawan, N.J., assignor to Hanson-Van Winkle-Mulching Company, a corporation of New Jersey No Drawing. Filed July 31, 1959, Ser. No. 830,719 11 Claims. (Cl. 204-141) This invention relates to the surface treatment of aluminum and, more particularly, to a process for electrolytically cleaning articles of aluminum and aluminum base alloys in an alkaline electrolyte. The invention proyides an improved process for cleaning articles of aluminum or its alloys in an alkaline bath using low-voltage alternating current electrolysis.

The cleansing of aluminum and aluminum alloys prior to finishing is regarded in the electroplating industry as essential to obtain satisfactory appearance and corrosion resistance in the finished article. Most of the oil and grease present on a dirty surface can be removed by solvent extraction or vapor degreasing, but, in general, solvent extraction by itself is inadequate for the preparation of an aluminum surface prior to finishing. Further chemical cleaning steps, usually following the degreasing stage, are almost invariably employed, the common purpose being to alter the surface of the object undergoing processing.

In general, these cleaning steps are based on immersing the aluminum article into either an acidic or an alkaline bath, the acidic baths usually being operated at or near the boiling point of the bath, while the alkaline baths are generally operated at much lower temperatures. The use of an acidic bath generally produces a bright surface, whereas the use of an alkaline cleanser is primarily designed to result in some form of etching.

Both chemical cleansing processes (acidic and alkaline) have been modified by passing direct current through the bath while the aluminum article is immersed, the workpiece being the anode (or positive) during this electrolytic cleaning, but these modifications have been found to be relatively time consuming and expensive. Uninhibited alkaline cleaning solutions generally attack aluminum rapidly, destroying polished finishes. The use of inhibited alkaline cleaners, however, frequently results in the formation of a surface film which hampers subsequent anodizing. If any movement occurs in an alkaline electrolyte during the operation, as it generally does be cause of gassing at the cathode, the surface of the aluminum workpiece which is the anode in direct current electrolysis frequently becomes marred by haze.

Ideally, an aluminum cleaner employing electrolysis should rapidly and effectively remove light oil, polishing compounds, or grease from the workpiece without materially altering the appearance of its surface, except perhaps to enhance the brightness of the surface finish. Such cleaners should be insensitive to any movement in the electrolytic bath and produce no surface films which might tend to interfere with subsequent processing, such as anodizing. Moreover, process economics dictate that any aluminum cleaner using an electrolytic treatment should be both inexpensive and yet be free from any unusual hazards or excessive corrosive conditions.

The present invention provides an improved process for electrolytically cleaning both aluminum and aluminum base alloys which fulfills every one of these requirements.

Using an aqueous alkaline electrolytic solution in which there is dissolved at least one alkali metal salt of a weak inorganic acid, I have found that by immersing an aluminum workpiece or article in such bath and making it an electrode in an alternating current circuit, it is possible to completely clean the aluminum surface without materially altering its surface finish by passing a relatively low-voltage alternating current through the immersed article for a. period ranging from five seconds to two minutes. Although effective cleaning by this method is possible using a wide range of bath compositions, the most effective cleaning and optimum economy of operation are obtained when the concentration of the alkaline salt in the bath is maintained in the range between 10 and 400 grams per liter.

Basically, the improved process of the invention for electrolytically cleaning articles of aluminum and aluminum base alloys comprises immersing the article in an aqueous alkaline electrolytic solution having a pH between 7.0 and 13.0 and in which there is dissolved from 10 to 400 grams per liter of at least one alkali metal salt of a weak inorganic acid, making the article an electrode in an alternating current circuit, and then passing alternating electric current through the immersed article for a period from 5 to 120 seconds and at a current density from 20 to 300 amperes per square foot while maintaining the temperature of the electrolytic solution between 45 C. and Under these conditions, the aluminum workpiece is completely cleaned from any adhering dint or contaminant, generally without materially altering its surface finish, except perhaps to brighten it at the higher current densities.

While any alkali metal salt of a weak inorganic acid may be selected for inclusion in the electrolytic bath, particularly satisfactory results have been obtained by using the alkali metal carbonates, phosphates, borates, and sulfites, including various combinations of these salts. Although these alkali metal salts may be used over a vary wide range of concentrations, in general ranging from 10 to 400 grams per liter, there is no great advantage in'using more than about 200 grams per liter, and in most aluminum cleaning solutions substantially the full benefit of their presence is achieved with grams per liter or even less.

Depending upon the particular operating conditions employed, the same electrolytic solution may be used to clean polished aluminum without aifecting its polished finish, or to clean unfinished aluminum strip while brightening its surface. For example, light deposits of oil and polishing compounds may be completely removed from a polished aluminum article without materially affecting its surface :by using current densities ranging from 40 to 50 amperes per square foot, a bath temperature between 45 C. and 60 C., an immersion (or electrolysis) period from 30 to 60 seconds, and a bath pH in the range from 10.5 to 12.5. On the other hand, by operating the electrolytic cleaning process at the higher current densities, generally in the range from 150 to 300 amperes per square foot, it is possible to both clean and brighten unfinished aluminum strip when the bath is operated at temperatures between 60 C. and 85 C. and at a pHin the range between 7.0 and 12.5, using an immersion period form 5 seconds (in the case of wire made from aluminum alloy 5052) to as long as seconds. Table I summarizes the optimum operating conditions for electrolytically cleaning aluminum and aluminum base alloys with a variety of cleaning solutions.

TABLE 1 Optimum Conditions for Electrolytically Cleaning Aluminum and Aluminum Base Alloys Gone. Bath Current Time Electrolyte (gm/1.) Temp. Density Volts (min.) pH Remark C.) (a.s.t.)

i Q 100-200 50-60 40-50 5. 5 0.5-1 10.5 Cleaning. KBCOL 150 50-00 40-50 1. 5 0. 5-1 11-13 Do.

150 50-60 150+ 6+ 0.5-1 11-13 Cleaning and brightening. N 2 3+ 2 03--.. 80 2 5 58:60 :-50 2 37 gsfi 11 Cleaging.

5 00 0-50 .5 .5- 11-1.5 0. NaCOa+Na3PO4"" 80-160 60-80 150+ 6+ 0.5-1 11-l1.5 Cleaning and brightening. i e r 100 58-28 150-t 8.5-1 8 13 01 Do.

5 40-5 .5-1 .5-1 eaning. Na2so3+NaOH 100-200 70-80 150-300 9+ 0.5-1 8.5-13 Cleaning and brightening.

l"he following examples are illustrative of the applicability of electrolytically cleaning articles of aluminum and An aqueous alkaline electrolytic cleaning solution having a pH of about 10.5 was prepared by dissolving grams per liter of sodium carbonate in water. After warming the solution to a temperature of 50 C., an aluminum test article coated with a thin film of light oil was immersed in the bath, made an electrode in an alternating current circuit (the other electrode being steel), and alternating electric current then passed through the immersed workpiece for a period of 30 seconds, using a current density of 40 amperes per square foot. Upon removal trom the electrolytic solution, the aluminum workpiece was completely cleansed of the adhering oil film without having been etched or marred by an appreciable amount of haze. As the concentration of sodium carbonate in the electrolytic solution was increased, the rate of cleaning of the aluminum workpiece became more rapid and the amount of haze diminished, the optimum results being obtained when the solution contained from 100 to 200 grams per liter of sodium carbonate. In another series of tests, identical results were obtained when potassium carbonate was substituted for sodium carbonate in the electrolytic cleaning solution.

EXAMPLE 11 Using an aqueous alkaline electrolytic solution having a pH of about 11 and containing 40 grams per liter of potassium carbonate and grams per liter of sodium carbonate, a polished aluminum workpiece which had been coated with a film of buffing compound was thoroughly cleaned without marring its polished appearance byimmersing the workpiece in the solution and passing alternating current through it for seconds, using a current density ranging from 40 to 50 amperes per square foot. The bath temperature was maintained at 57 'C. to 58 C., and no agitation was provided.

EXAMPLE in Aqueous electrolytic solutions of trisodium phosphate are very elfective for removing light oils from aluminum. Using an aqueous solution of 100 grams per liter of trisodium phosphate, which was buifered to a "pH of 12.5 by the addition of 25 grams per liter of monosodium phosphate, a polished aluminum workpiece which was deliberately contaminated with a thin layer of light oil was thoroughly cleaned while retaining its polished brilliance by immersing the workpiece in the bath and then passing alternating electric current through the immersed workpiece at a current density of 40 amperes per square --foot for a period of 30 seconds, 'while maintaining the temperature of the bath at 55 C.

EXAMPLE IV Aqueous solutions of sodium carbonate and trisodium phosphate are especially efiective for both light-duty and heavy-duty cleaning. An aluminum test article which had previously been coated with a film of light oil was immersed in an aqueous alkaline electrolytic solution containing 50 grams per liter of sodium carbonate and 30 grams per liter of trisodium phosphate, the temperature of the bath being maintained at 50 C. The immersed test article was made an electrode in an alternating current circuit, and alternating electric current was then passed through it for a period of 30 seconds, using a current density of 45 amperes per square foot. Upon removal from the bath, the aluminum workpiece was found to be completely free from the oil film, although its surface finish was materially unaltered by the electrolytic treatment.

Excellent cleaning was also obtained when the aluminum workpiece was coated with a :heavy oil, in which case the bath should contain 100 grams per liter of sodium carbonate and 60 grams per liter of trisodium phosphate for optimum results and operating economy.

EXAMPLE V At the higher current densities, generally from 150 to 300 amperes per square foot, the electrolytic cleaning of aluminum or its' alloys in solutions containing sodium borate is usually accompanied by a pronounced brightening effect. Using an aqueous alkaline electrolytic solution of 100 grams per liter of sodium borate, to which lrad been added a sufficient amount o f sodium hydroxide to bring the pH to 13, a panel of aluminum alloy 1100 which had previously been coated with a film of light oil was thoroughly cleaned by immersing the panel in the solution, making it one of the electrodes in an alternating current circuit, and then passing alternating electric current through the immersed panel at a current density of 150 amperes per square foot for a period of one minute. The temperature of the solution was maintained at 70 C. throughout the electrolysis. Upon removal from the bath, the panel was found to be completely free from all adhering oil films and to possess a notably brighter surface than it did prior to the electrolytic treatment.

EXAMPLE VI Using an aqueous alkaline electrolytic solution maintained at a temperature of C. and which contained 200 grams per liter of sodium sulfite, to which had been added 2 grams per liter of sodium hydroxide to adjust the pH to about 13, an unpolished panel of aluminum alloy 1100 coated with a fil-m of dirt was immersed in the hath, made an electrode in an alternating current circuit, and alternating electric current then passed through the immersed panel for a period of two minutes, using a current density of 300 amperes per square foot. Upon removal from the bath, the panel was completely cleaned of the dirt film and possessed a very bright appearance.

EXAMPLE VII Using an aqueous solution containing grams per liter of sodium carbonate and 60 grams per liter of trisodium phosphate (monohydrate), in which the pH had been adjusted to about 1l;0 by the addition of phosphoric acid, polished aluminum tubing of 1-100 alloy was satisfactorily cleaned without significant haze formation at a current density of about 20 amperes per square foot applied for one minute at a temperature of 55 C. The cleaning solution was moderately agitated by moving the aluminum tubing being cleaned.

I claim:

1. A process for electrolytically cleaning articles of aluminum and aluminum base alloys without degrading their surface finish which comprises immersing the article in an aqueous alkaline electrolytic solution having a pH between 7.0 and 13.0 and in which there is dissolved from to 400 grams per liter of at least one alkali metal salt of a weak inorganic acid, making the article an electrode in an alternating current circuit, and passing alternating electric current through the immersed article for a period from 5 to 120 seconds and at a current density from to 300 amperes per square foot while maintaining the temperature of the electrolytic solution between 45 C. and 85 C.

2. A process for electrolytically cleaning articles of aluminum and aluminum base alloys without degrading their surface finish which comprises immersing the article in an aqueous alkaline electrolytic solution having a pH between 10.5 and 12.5 and in which there is dissolved from 20 to 200 grams per liter of at least one alkali metal salt of a weak inorganic acid, making the article an electrode in an alternating current circuit, and passing alternating electric current through the immersed article for a period from 30 to 60 seconds and at a current density from 20 to 50 amperes per square foot while maintaining the temperature of the electrolytic solution between 45 C. and 60 C.

3. A process for electrolytically cleaning articles of aluminum and aluminum base alloys without degrading their surface finish which comprises immersing the article in an aqueous alkaline electrolytic solution having a pH between 10.5 and 12.5 and in which there is dissolved from 20 to 200 grams per liter of at least one alkali metal salt selected from the group consisting of carbonates, phosphates, borates, and sulfites, making the article an electrode in an alternating current circuit, and passing alternating electric current through the immersed article for a period from 30 to 60 seconds and at a current density from 20 to 50 amperes per square foot while maintaining the temperature of the electrolytic solution between 45 C. and 60 C.

4. A process for electrolytically cleaning articles of aluminum and aluminum base alloys without degrading their surface finish which comprises immersing the article in an aqueous alkaline electrolytic solution having a pH between 10.5 and 12.5 and in which there is dissolved from 20 to 200 grams per liter of at least one alkali metal carbonate, making the article an electrode in an alternating current circuit, and passing alternating electric current through the immersed article for a period from 30 to 60 seconds and at a current density from 20 to 50 amperes per square foot while maintaining the temperature of the electrolytic solution between 45 C. and 60 C.

5. A process for electrolytically cleaning articles of aluminum and aluminum base alloys without degrading their surface finish which comprises immersing the article in an aqueous alkaline eletrolytic solution having a pH between 10.5 and 12.5 and in which there is dissolved from 100 to 200 grams per liter of sodium carbonate, making the article an electrode in an alternating current circuit, and passing alternating electric current through the immersed article for a period from 30 to 60 seconds and at a current. density from 0 to 50 amperes per square foot while maintaining the temperature of the electrolytic solution between 50 C. and 60 C.

6. A process for electrolytically cleaning articles of aluminum and aluminum base alloys without degrading their surface finish which comprises immersingthe article in an aqueous alkaline electrolytic solution having a pH between 10.5 and 12.5 and in which there is dissolved from 100 to 200 grams per liter of potassium carbonate, making the article an electrode in an alternating current circuit, and passing alternating electric current through the immersed article for a period from 30 to 60 seconds and at a current density from 20 to 50 amperes per square foot while maintaining the temperature of the electrolytic solution between 50 C. and 60 C.

7. A process for electrolytically cleaning articles of aluminum and aluminum base alloys without degrading their surface finish which comprises immersing the article in an aqueous alkaline electrolytic solution having a pH between 10.5 and 12.5 and in which there is dissolved from 40 to grams per liter of potassium carbonate and from 25 to 50 grams per liter of sodium carbonate, making the article an electrode in an alternating current circuit, and passing alternating electric current through the immersed article for a period from 30 to 60 seconds and at a current density from 20 to 50 amperes per square foot while maintaining the temperature of the electrolytic solution between 50 C. and 60 C.

8. A process for electrolytically cleaning articles of aluminum and aluminum base alloys without degrading their surface finish which comprises immersing the article in an aqueous alkaline electrolytic solution having a pH between 10.5 and 12.5 and in which there is dissolved from 10 to 125 grams per liter of at least one alkali metal phosphate, making the article an electrode in an alternating current circuit, and passing alternating electric current through the immersed article for a period from 30 to 60 seconds and at a current density from 20 to 50 amperes per square foot while maintaining the temperature of the electrolytic solution between 45 C. and 60 C.

9. A process for electrolytically cleaning articles of aluminum and aluminum base alloys without degrading their surface finish which comprises immersing the article in an aqueous alkaline electrolytic solution having a pH between 10.5 and 12.5 and in which there is dissolved from 50 to grams per liter of sodium carbonate and 30 to 60 grams per liter of trisodium phosphate, making the article an electrode in an alternating current circuit, and passing alternating electric current through the immersed article for a period from 30 to 60 seconds and at a current density from 20 to 50 amperes per square foot while maintaining the temperature of the electrolytic solution between 45 C. and 60 C.

10. A process for electrolytically cleaning articles of aluminum and aluminum base alloys without degrading their surface finish which comprises immersing the article in an aqueous alkaline electrolytic solution having a pH between 10.5 and 12.5 and in which there is dissolved from 20 to 200 grams per liter of at least one alkali metal sulfite, making the article an electrode in an alternating current circuit, and passing alternating electric current through the immersed article for a period from 30 to 60 seconds and at a current density from 20 to 50 amperes per square foot while maintaining the temperature of the electrolytic solution between 45 C. and 60 C.

11. A process for electrolytically cleaning articles of aluminum and aluminum base alloys without degrading their surface finish which comprises immersing the article in an aqueous alkaline electrolytic solution having a pH between 8.5 and 13.0 and in which there is dissolved from 20 to 200 grams per liter of an alkali metal borate, making the article an electrode in an alternating current circuit, and passing alternating electric current through the immersed article for a period from 5 to seconds and at a current density from to 300 amperes per square foot while maintaining the temperature of the electrolytic solution between 50 C. and 70 C.

(References on following page) Rfepences Iited in thgjfile of this patent *I N D STATES PAT 8 Patrie May 22, 1951 Freud- Aug. 4, 1953 HeSch June 29, 1954 Pattie May 15,1956 Mostovych June 21, 1960 

1. A PROCESS FOR ELECTROLYTICALLY CLEANING ARTICLES OF ALUMINUM AND ALUMINUM BASE ALLOYS WITHOUT DEGRADING THEIR SURFACE FINISH WHICH COMPRISES IMMERSING THE ARTICLE IN AN AQUEOUS ALKALINE ELECTROLYTIC SOLUTION HAVING A PH BETWEEN 7.0 AND 13.0 AND IN WHICH THERE IS DISSOLVED FROM 10 TO 400 GRAMS PER LITER OF AT LEAST ONE ALKALI METAL SALT OF A WEAK INORGANIC ACID, MAKING THE ARTICLE AN ELECTRODE IN AN ALTERNATING CURRENT CIRCUIT, AND PASSING ALTERNATING ELECTRIC CURRENT THROUGH THE IMMERSED ARTICLE FOR A PERIOD FROM 5 TO 120 SECONDS AND AT A CURRENT DENSITY FROM 20 TO 300 AMPERES PER SQUARE FOOT WHILE MAINTAINING THE TEMPERATURE OF THE ELECTROLYTIC SOLUTION BETWEEN 45*C. AND 85*C. 